1 Field of the Invention
The present invention relates to an apparatus for separating a fluid flow, more particularly an apparatus for extracting one or more heavier fractions from a fluid mixture being carried through a pipeline.
2 Technical Background of the Invention
In the production of oil, for example, it is very important that water is separated from the oil in an efficient manner. By separating water from the oil in the area of a well, the water may be re-injected or processed in the near well area. In this manner, the need for energy for transporting the oil to the platform, floats, or land-based installations is reduced. In addition, problems associated with emulsions and precipitations in the transport pipeline are reduced.
The government has made demands regarding the amount of oil contained in water discharged from permanent installations, the maximum allowed value today being 40 mg H-C/l water (H—C=Hydrocarbons). The maximum allowed values for floating installations are the same as for ships, i.e. approx. 100 mg H-C/l water. In other applications there is a need for separating water and/or oil from gas. In the production of gas, it is important to dehydrate the gas in order to avoid that droplets cause damage to compressors used for pressurizing the gas for transport through pipelines. At high pressures the gas dehydration process is complicated in that 1) there is only a small density difference between the liquid and gas, and 2) the interface tension between the liquid and gas is small. It is therefore critical that the fluid shear stress of the separated liquid flow is minimized. The present invention is suitable for all the above applications.
Today, various methods exist for the processing of water containing oil or wet gas, which methods may be broadly subdivided into the following groups:                separation by means of gravity,        separation by filtering,        separation by floatation, and        separation by acceleration.        
The present invention belongs to the latter group.
WO 01/002967 A1 relates to a device for separating a fluid flow comprising a lighter and a heavier fraction, wherein the fluid flows through a pipeline and is made to rotate by a rotary element in the fluid path. The device operates according to the cyclone principle as the rotary motion of the fluid forces the heavier fluid fractions outwards whereas the lighter fractions remain close to the center of the pipe bore. At the center of the pipe bore a perforated pipe body is arranged through which the lighter fractions are guided.
U.S. Pat. No. 5,062,955 relates to a hydrocyclone separating high and low density fluids. The cyclone is driven by a motor and the incoming fluid is separated into three fractions, namely water, diluted oil and concentrated oil.
Another approach that may be used for acceleration separation is the centrifuge principle. The operating principle of a centrifuge is considered well known and needs no further explanation. Generally, this principle will be suited only for mixtures of liquids or mixtures of liquid and solids, perhaps liquids containing dispersed gases.
One of the drawbacks of the above prior art technologies is that the separation effect is not as good as could be desired. One of the reasons for this is that the relative difference in rotational velocity between the fluids within the pipe bore and the inner surface of the pipe bore results in fluid shear stresses causing the more or less continuous film of the heavier fluids accumulated on the inner surface of the pipe bore to be dragged loose and carried along with the fluid at the pipe bore center (entrainment). This phenomenon deteriorates the separation effect.